The archaea and bacteria aggregate to form “consortia” in marine and freshwater sediments that are low in oxygen but high in methane and sulfur.
The consortium performs “anaerobic oxidation of methane” (AOM), a process that provides energy for the consortium and drastically decreases the amount of methane released into the atmosphere. (Methane is a powerful greenhouse gas, as explained by the video below.)
Orphan and her team knew that microbes in the consortium worked together to perform AOM, and they predicted that cells in the consortium would be organized in a way that allowed them to share metabolites by simple diffusion. That is, metabolites would essentially drift between cells. The team used an isotope tracer to determine which cells within the consortium were metabolically active. What they found, was that the microbes shared metabolites effectively regardless of their spatial organization. In other words, it appeared that metabolites were being shared between cells across relatively long distances, a finding that couldn’t be explained by simple diffusion. The team turned to computer modeling and verified that the only metabolites able to cross relatively long distances were electrons.
While interspecies electron transfer had never been observed, electron transfer among the genus Geobacter is well characterized. These bacteria use extracellular multi-heme cytochrome (MHC) proteins to transport electrons between cells. Orphan and her team searched for, and found, MHC-like genes in their methane-oxidizing archaea. (Only a few methane-oxidizing archaea genomes are sequenced because they are difficult to grow. Some species only reproduce four times a year!) They also used electron microscopy and a diagnostic stain for MHCs to show that, like Geobacter, their methane-oxidizing archaea expressed cell-surface MHCs.
Taken together, Orphan proposes a model in which methane is oxidized and the resulting electrons are transferred to extracellular MHC proteins. From there, the electrons confer conductivity to the extracellular matrix separating the methane-oxidizing bacteria and their sulfate-reducing partners.
Sources: Caltech, Nature, Wikipedia